The present invention relates to an electric power steering apparatus in which an electric motor gives steering assist force to a steering mechanism of a vehicle and, more specifically, to the reduction of torque ripples of the electric motor of an electric power steering apparatus.
Electric power steering apparatus in which steering assist force is given to a steering mechanism by driving an electric motor in accordance with steering torque that is applied to a steering wheel by a driver are used conventionally.
Incidentally, in electric motors, ripples necessarily occur in output torque. And the torque ripple is generally classified into one (called “mechanical ripple” or “cogging torque”) that is caused by structural factors such as the number of poles and the number of slots of the motor and one (hereinafter referred to as “electric ripple”) that is caused by deviation of an induced electromotive force waveform of the motor from its ideal waveform (a DC waveform in the case of a motor with a brush or a sinusoidal wave (for each phase) in the case of a 3-phase-sinusoidal-wave-driven brushless motor). In electric power steering apparatus, a complaint is sometimes made about deterioration in steering feeling due to torque ripples. And motor makers are making efforts to minimize torque ripples. However, of the above kinds of torque ripples, the electric ripple appears as ripples whose magnitude is proportional to the motor load (i.e., motor current or output torque) and hence is particularly problematic in electric power steering apparatus in which importance is attached to the smoothness of the motor output torque.
In the above circumstances, methods for reducing torque ripples by making improvements in the motor driving method were proposed in the art. For example, to reduce torque ripples in a 3-phase-sinusoidal-wave-driven brushless motor, a technique was proposed (refer to non-patent document 1) in which optimum d-axis and q-axis current waveforms (having periodic variations) for canceling out electric ripples due to distortion of induced electromotive force waveforms are determined on the basis of measurement data of no-load induced electromotive force (waveforms of those data are distorted from sinusoidal waves) This technique makes it possible to almost eliminate torque ripples in a constant-speed rotation state even with a load variation by increasing the amplitudes of compensation currents having the thus-determined d-axis and q-axis current waveforms in proportion to the load.
Patent Document 1
JP-A-11-191992
Non-patent Document 1
Yoshitada Chin and Takashi Sekiguchi, “High-Efficiency, Low-Torque Control of Permanent Magnet Synchronous Motor Using a Current that Follows Induced Electromotive Force Vectors,” The Transactions D of The Institute of Electrical Engineers of Japan, The Institute of Electrical Engineers of Japan, Vol. 120, No. 4, pp. 559-565, 2000.
However, in motors used in electric power steering apparatus, the rotation speed varies every moment depending on not only the load variation but also the drive situation. Therefore, electric ripples cannot be reduced sufficiently by applying the above technique as it is to an electric power steering apparatus.